Implement signed API data type whitelisting

[bbenligiray]

The signed API accepts and serves any signed data from any Airnode address. It may want to be selective about this. There can be two features:

• Only accept signed data from specific Airnode addresses
• Only accept signed data from specific Airnode addresses for specific templates

[Siegrift]

Only accept signed data from specific Airnode addresses

This was done as part of #101

Only accept signed data from specific Airnode addresses for specific templates

This one is not yet supported.

[Siegrift]

Only accept signed data from specific Airnode addresses for specific templates

This one is not yet supported.

It's nice to have this fine grain support, but it's also very hard to maintain. We agreed on call to leave it unimplemented for now.