sshd: try all configured signature algorithms for a key

sshd: try all configured signature algorithms for a key

For RSA keys, there may be several configured signature algorithms:
rsa-sha2-512, rsa-sha2-256, and ssh-rsa. Upstream sshd has bug
SSHD-1105 [1] and always and unconditionally uses only the first
configured algorithm. With the default order, this means that it cannot
connect to a server that knows only ssh-rsa, like for instance Apache
MINA sshd servers older than 2.6.0.

This affects for instance bitbucket.org or also AWS Code Commit.

Re-introduce our own pubkey authenticator that fixes this.

Note that a server may impose a penalty (back-off delay) for subsequent
authentication attempts with signature algorithms unknown to the server.
In such cases, users can re-order the signature algorithm list via the
PubkeyAcceptedAlgorithms (formerly PubkeyAcceptedKeyTypes) ssh config.

[1] https://issues.apache.org/jira/browse/SSHD-1105

Bug: 572056
Change-Id: I7fb9c759ab6532e5f3b6524e9084085ddb2f30d6
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>