GC-45: Update copyright information:

- Changes to code base were previously done in 2020 as well (based on
git history).

GC-37: Use KeyGenerator instead of generate_private_key in unit tests:

- Replaced all uses of generate_private_key with KeyGenerator
invocations that explicitly request 2048-bit RSA.

GC-37: Added support for requesting custom RSA key size when issuing client certificates:

- Added functional test.
- Added unit tests.
- Added new CLI option to the client command.
- Updated the client command to use the passed-in key specification.
- Updated existing tests to cope with changes to the client command
function signature.
- Fixed small typo in docstring for the server command.
- Fixed functional test for client command help that expected
positional argument in a specific line.

GC-37: Added support for requesting custom RSA key size when issuing server certificates:

- Added functional test.
- Added unit tests.
- Added new CLI option to the server command.
- Updated the server command to use the passed-in key specification.
- Updated existing tests to cope with changes to the server command
function signature.

GC-37: Refactor key specification handling:

- Perform the key specification parsing within CLI module itself,
don't do it via crypto module.
- Pass-in tuple consisting out of algorithm and associated parameters
into the init command instead of key generator.
- Updated all tests to accomodate the change in init function
signature.
- Simplify the KeyGenerator class.
- Do not test if KeyGenerator class sets the properties via
constructor - it is sufficient to test string represenation and key
generation.

GC-37: Added support for requesting custom RSA key size when initialising the CA hierarchy:

- Added functional test.
- Added unit tests.
- Added new CLI option for specifying the algorithm.
- Implemented KeyGenerator factory-like class that can be called to
generate a private key with desired specification.
- The init init function now accepts a callable that is used to
generate private keys.
- The generate_ca_hierarchy function now accepts a callable that is
used to generate private keys.
- Updated existing unit tests to cope with changes to the init
and generate_ca_hierarchy function signatures.
- Updated existing unit tests to cope with changes to existing
functionality.
- Updated existing functional tests to cope with changes in command
output.

GC-37: Introduce gctmpdir fixture for reducing duplication in tests:

- Fixture can be used to initialise the temporary directory with
1-level deep Gimmecert hierarchy. It is very useful for tests that
do not care about hierarchy details, while at the same time being
much faster than the sample directory one.
- Fixture should not be used for testing of init/status
commands (since those heavily test what the hierarchy looks like).

GC-23: Removed option to update DNS names from server command:

- Removed functional test.
- Dropped the option from CLI.
- Dropped the option from command function implementation.
- Updating code for new command function signature.
- Updated existing unit tests.
- Removed unneeded unit tests.

GC-22: Updated server command to allow reading of CSR from stdin:

- Implemented an additional helper for functional tests for running
interactive commands.
- Implemented functional test covering passing-in CSR to the server
command via stdin (interactively).
- Updated server command implementation.
- Implemented utility function for reading input from user.
- Implemented utility function for reading CSR from string in (in
OpenSSL-style PEM format)
- Fixed some missing imports in the custom pytest fixture.
- Implemented relevant unit tests.

GC-22: Refactored server command tests for testing output on success:

- Introduced custom pytest fixture that sets-up a small Gimmecert
project.
- Introduced custom pytest fixture that sets-up private key with CSR.
- Replaced all server command tests that check the resulting output
with a parametrised test. One test should actually fail, but this is
a bug in implementation. Will fix in subsequent commit.
- Introduced separate tests that ensure the server private key or CSR
do no get overwritten in case DNS name update is requested.