Support pgp trust levels

[danielemery]

PGP keys use a trust model, with levels such as Ultimate, Trusted, Untrusted etc.

If they keys project will be used as a method to store trusted keys as well your own keys, the concept of trust should be considered.

~It's proposed that within the keys folder there can be optional subfolders, one for each trust level.~

• ~If at least one of these exist keys in the root folder will be considered Untrusted otherwise keys will be considered Ultimate~
• ~Keys in the subfolders will be treated with the trust level of the folder~
• ~The /pgp endpoint will be updated to allow a query argument for ?trust to allow filtering on trust~

[danielemery]

Since https://github.com/danielemery/keys/issues/51 there was a pivot to use the config file to mount pgp keys. This means:

• This issue would no longer be considered a breaking change (so we will remove the v2 milestone)
• We can simply use new optional fields in the config file to specify trust levels