signer: add constants for key types and signing schemes

[lukpueh]

Supported key types and signing scheme strings are currently hardcoded all over the library. We should define them in a single place e.g. securesystemslib.signer._constants and update all secureystemslib.signer.* modules to use them. (I suggest to not touch legacy modules such as secureystemslib.keys)

Publicly registered key types and schemes can be found in:

securesystemslib/securesystemslib/signer/__init__.py

Lines 39 to 55 in 135567f

	("ecdsa", "ecdsa-sha2-nistp256"): SSlibKey,
	("ecdsa", "ecdsa-sha2-nistp384"): SSlibKey,
	("ecdsa-sha2-nistp256", "ecdsa-sha2-nistp256"): SSlibKey,
	("ecdsa-sha2-nistp384", "ecdsa-sha2-nistp384"): SSlibKey,
	("ed25519", "ed25519"): SSlibKey,
	("rsa", "rsassa-pss-sha224"): SSlibKey,
	("rsa", "rsassa-pss-sha256"): SSlibKey,
	("rsa", "rsassa-pss-sha384"): SSlibKey,
	("rsa", "rsassa-pss-sha512"): SSlibKey,
	("rsa", "rsa-pkcs1v15-sha224"): SSlibKey,
	("rsa", "rsa-pkcs1v15-sha256"): SSlibKey,
	("rsa", "rsa-pkcs1v15-sha384"): SSlibKey,
	("rsa", "rsa-pkcs1v15-sha512"): SSlibKey,
	("sphincs", "sphincs-shake-128s"): SpxKey,
	("rsa", "pgp+rsa-pkcsv1.5"): GPGKey,
	("dsa", "pgp+dsa-fips-180-2"): GPGKey,
	("eddsa", "pgp+eddsa-ed25519"): GPGKey,

Additionally, there are a few non-registered ones, which ca be found by looking for implementations of the Key interface.

[lukpueh]

The 3 original key types are already defined as constants in init, which are only used in securesystemslib.interface. I suggest to deprecate them in favour of constants defined in the signer sub-package.