ReceivePack: Receive and parse client session-id.

ReceivePack: Receive and parse client session-id.

Before this change JGit did not support the session-id capability
implemented by native Git. This change implements advertising the
capability from the server and parsing the session-id received from
the client during a ReceivePack operation.

Enable the transfer.advertisesid config setting to advertise the
capability from the server. The client may send a session-id capability
in response. If received, the value from this is parsed and available
via the getClientSID method on the ReceivePack object. All capabilities
in the form `capability=value` are now split into key value pairs at the
first `=` character. This change replaces specific handling for the
agent capability.

This change does not add advertisement or parsing to UploadPack. This
change also does not add the ability to send a session ID from the JGit
client.

https://git-scm.com/docs/protocol-v2/2.33.0#_session_idsession_id

Change-Id: I56fb115e843b11b27e128c4ac427b05d5ec129d0
Signed-off-by: Josh Brown <sjoshbrown@google.com>