* ChainSpec trait

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>

* Added docs

* Fixed build

* Fixed build

Co-authored-by: Bastian Köcher <[email protected]>

* Use CLI to configure max instances cache

* Fix tests

* Move default value into CLI

* Use SmallVec

* Apply review comments

* Get rid of `SmallVec`

Co-authored-by: Bastian Köcher <[email protected]>

The only up-to-date deployment of PolkadotJS apps is currently on https://polkadot.js.org/apps/.
https://substrate-ui.parity.io is only useful as deliberately kept outdated version of Apps, to be used for projects still on Substrate 1.0
It cannot be used with the up-to-date chains running the Substrate 2.0 codebase, and probably won't be maintained much longer.

* Add a few metrics to Client

* Improve PrometheusConfig

* Fix client docs

* Make export blocks default to json on stdout

* Multiline instead of single line to stay under 100 cols

* Change --json flag to --binary, defaulting to json

* Initial commit

Forked at: 41bb2193
Parent branch: origin/master

* Increase killing grace period of CLI tests and display more info

* Use --dev everywhere possible

* Put pruning mode to its own params struct

* Add pruning params to export-blocks command

* Added missing file

* Removed not-dev mode in tests

* Add pruning mode to the revert command

* Decrease killing grace period again

* Move back unsafe_pruning to import_params

* Applied proposed changes

Related to #4776 
Related to https://github.com/paritytech/polkadot/pull/832

To summarize the changes:
1. I did not manage to validate with types the service's Configuration. But I did reduce the possibility of errors by moving all the "fill" functions to their respective structopts
2. I split params.rs to multiple modules: one module params for just CLI parameters and one module commands for CLI subcommands (and RunCmd). Every command and params are in their own file so things are grouped better together and easier to remove
3. I removed the run and run_subcommand helpers as they are not helping much anymore. Running a command is always a set of 3 commands: 1. init 2. update config 3. run. This still allow the user to change the config before arguments get parsed or right after.
4. I added tests for all subcommands.
5. [deleted]

Overall the aim is to improve the situation with the Configuration and the optional parameters, add tests, make the API more consistent and simpler.