Define accepted log public keys

[sleevi]

The Chromium implementation of CT is limited in support of public keys to the set of public keys it accepts for the Web PKI:

• RSA (nominally, 2048, 3072, 4096)
• ECC using NIST P-256, P-384

The Chromium implementation does not explicitly support Curve25519, although it could, and does not support other forms of EC keys.

[devonobrien]

We also need to check what key algs our compliance monitoring infrastructure supports to provide the minimal set of supportable key types.

[devonobrien]

Also, of note is Section 2.1.4 of RFC 6962 which states:

Various data structures are signed. A log MUST use either elliptic
curve signatures using the NIST P-256 curve (Section D.1.2.3 of the
Digital Signature Standard [DSS]) or RSA signatures (RSASSA-PKCS1-
V1_5 with SHA-256, Section 8.2 of [RFC3447]) using a key of at least
2048 bits.