validate Key instances in constructor not in deserialization

[lukpueh]

Description of issue or feature request:
Subclasses of Key implement a from_dict deserialization interface to load known keys e.g. from TUF metadata. Some of the current implementations do basic input validation too.

Unless it is directly related to deserialization, I think, input validation should happen in __init__, to also be effective on manually created objects.

Examples:

securesystemslib/securesystemslib/signer/_key.py

Lines 150 to 151 in 09e092e

	if "public" not in keyval or not isinstance(keyval["public"], str):
	raise ValueError(f"public key string required for scheme {scheme}")

securesystemslib/securesystemslib/signer/_sigstore_signer.py

Lines 42 to 46 in 09e092e

	for content in ["identity", "issuer"]:
	if content not in keyval or not isinstance(keyval[content], str):
	raise ValueError(
	f"{content} string required for scheme {scheme}"
	)

Current behavior:
Key implementations validate inputs in from_dict

Expected behavior:
Key implementations validate inputs in __init__