continuous PGF proposals should have beginning and end epoch options

[gavinly]

https://docs.namada.net/users/governance/proposal-types#pgf-proposal

if i want to have continuous NAM sent to a recipient, i may want advanced directives so that the PGF begins at a certain epoch and ends at another epoch. having these options would enable PGF to have finer control without needing further governance proposals

[gavinly]

@brentstone could this be ready by March?

[brentstone]

@gavinly I think it is possible that it could be.