Allow configurable keyring (collection) to use

[camh-]

Make the keyring/collection used to store the git secrets configurable via the usual git config.

The keyring may be an alias, which is what the "default" keyring is, or an actual collection. If it is not an existing alias and not a collection, we should create the collection for the given name. This allows the user to store all their git credentials in a separate keyring if they want and it will be created automatically if it does not exist.

[camh-]

The collection will be specified as --collection <string> or -c <string> and applies to the three operatons (get, store, delete). The flag will be at the program level so it can be applied as git-credential-fdoss --collection foo store ... as the configuration of the helper would place the flags before the operation. That is, one can specify the helper in the git config as:

[credential]
    helper = fdoss --collection foo

and git will append the operation (get, store, delete) to the command. This means the flag cannot be at the operation level as those must be specified after the operation (sub-command) on the command line.

If the collection is not specified (empty string), then credentials are stored in the collection with the default alias, and get and delete look for the credential across all collections. The is the current behaviour.

If a collection is specified, then it will be used exclusively for the operation for which it is specified. If a credential is being stored and the specified collection does not exist, it will be created.