tls_credentials: add Trusted Credential storage backend

This adds a Trusted Credential storage backend using the PSA
Protected Storage API, permitting storage of the credentials
offering a trusted long-term storage with integrity checks.

This implementation tries to fit the actual tls_credentials
implementation, with some slight differences:
- the buffer pointer returned by credetial_get & credential_next_get
  is dynamically allocated and differs from the one given to
  tls_credential_add since it's extracted from the storage at runtime.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>