Filter transactions & accounts by metadata

[altitude]

Summary
We should be able to filter transactions by metadata

Solution proposal
GET /:ledger/transactions?meta:key=value
GET /:ledger/accounts?meta:key=value

The match should be done on an exact value.

Because the metadata object on transactions and accounts is always stored key by key, with the value stored as a json string we should be able to pass any type as value, scalar or compound.

Describe alternatives you've considered
Nothing yet, happy to discuss alternative ideas

Additional context
Follow-up of a discord discussion

[henry-jackson]

@altitude I just read through the discord discussion; I would be happy to work on this!

[jdupas22]

The correct format used to day to request metadata is this one:
metadata[key]=value1&metadata[a.nested.key]=value2