Add hostkey_algos to the git.ssh schema

[stefanprodan]

Description

Allow configuring the list of host key algorithms to use for SSH connections initialised by the provider during bootstrap.

Motivation and Context

Closes: #678

xref: fluxcd/flux2#4754

How has this been tested?

• Have you added an acceptance test for the functionality being added?
• Have you run the acceptance tests on this branch?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Documentation

• I have updated the documentation (if required) with make docs

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s