RPC front end implementation

[seancolsen]

## Phase 1 — POC
- [x] connections.add_from_known_connection
- [x] connections.add_from_scratch

## Phase 2 — D3L & DQL
- [ ] schemas.list
- [ ] tables.get
- [ ] tables.list
- [ ] tables.list_joinable
- [ ] columns.list
- [ ] constraints.list
- [ ] types.list
- [ ] records.list
- [ ] records.get

## Phase 3 — DML
- [ ] records.delete
- [ ] records.add
- [ ] records.patch

## Phase 4 — DDL
- [ ] schemas.delete
- [ ] schemas.add
- [ ] schemas.patch
- [ ] tables.delete
- [ ] tables.add
- [ ] tables.patch
- [ ] columns.delete
- [ ] columns.add
- [ ] columns.patch
- [ ] constraints.delete
- [ ] constraints.add

## Phase 5 — Metadata
- [ ] tables.metadata.list
- [ ] tables.metadata.patch
- [ ] columns.metadata.list
- [ ] columns.metadata.patch
- [ ] connections.list
- [ ] connections.delete
- [ ] connections.patch
- [ ] explorations.list
- [ ] explorations.get
- [ ] explorations.delete
- [ ] explorations.add
- [ ] explorations.replace
- [ ] explorations.run
- [ ] explorations.run_saved
- [ ] shared_explorations.list
- [ ] shared_explorations.add
- [ ] shared_explorations.patch
- [ ] shared_explorations.regenerate
- [ ] shared_tables.list
- [ ] shared_tables.add
- [ ] shared_tables.patch
- [ ] shared_tables.regenerate

## Phase 6 — Odds and ends
- [ ] tables.get_import_preview
- [ ] data_modeling.add_link
- [ ] data_modeling.move_columns
- [ ] data_modeling.split_table
- [ ] data_modeling.suggest_types

## Phase 7 — Users
- [ ] users.list
- [ ] users.get
- [ ] users.delete
- [ ] users.add
- [ ] users.patch
- [ ] users.password.replace_own
- [ ] users.password.revoke