The credentials callback may be passed a username in case where
the URL already includes the expected username. As we usually
cannot use a different username in such context, we should use
that one if provided and not ask the user for a diferent
username.

Implement SSH key credentials. This allows users to use the SSH
transport with the lg2 example code.

Implement SSH key credentials. This allows users to use the SSH
transport with the lg2 example code.

Implement plain username credential types. These type of
credentials might be asked for e.g. as some kind of
pre-authentication step, before the actual credentials are
passed.

Credential callback are being passed a bitset that indicates
which credential types are allowed in the current context. In our
examples code, we completely ignore that field and always return
username/password credentials, which doesn't necessarily make
sense e.g. when only SSH keys are allowed.

Refactor the code and only return username/password credentials
in the case where `USERPASS_PLAINTEXT` credentials are allowed.
Otherwise, return a positive error code to indicate that no
credentials could be acquired.

We currently have two locations in our examples where we define
the same compatibility wrappers for MSVC. Move them into
"common.h" to avoid duplication and make them available to other
examples.

Update internal usage of `git_transfer_progress` to
`git_indexer_progreses`.

Inside of our networking example code, we have a git2 executable
that acts as an entry point to all the different network
examples. As such, it is kind of the same like the normal git(1)
executable in that it simply arbitrates to the respective
subcommands.

Let's extend this approach and merge all examples into a single
standalone lg2 executable. Instead of building an executable
for all the existing examples we have, we now bundle them all
inside of the lg2 one and let them be callable via subcommands.

In the process, we can get rid of duplicated library
initialization, deinitialization and repository discovery code.
Instead of having each subcommand handle these on its own, we
simply do it inside of the single main function now.