jwt: reshape and add alternative for ECDSA using PSA

This commit:

- creates 2 new files, jwt_ecdsa.c and jwt_rsa.c, to hold the
  implementations of the corresponding ECDSA/RSA signature
  algorithms;
- RSA signature is stil done through Mbed TLS's PK module which
  can optionally make use of PSA (if enabled);
- ECDSA signature will instead use PSA, if possible, or TinyCrypt
  as fallback.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>