Introduce validator definition file for VC (#1357)

## Issue Addressed

NA

## Proposed Changes

- Introduces the `valdiator_definitions.yml` file which serves as an explicit list of validators that should be run by the validator client.
  - Removes `--strict` flag, split into `--strict-lockfiles` and `--disable-auto-discover`  
  - Adds a "Validator Management" page to the book.
- Adds the `common/account_utils` crate which contains some logic that was starting to duplicate across the codebase.

The new docs for this feature are the best description of it (apart from the code, I guess): 9cb87e93ce/book/src/validator-management.md

## API Changes

This change should be transparent for *most* existing users. If the `valdiator_definitions.yml` doesn't exist then it will be automatically generated using a method that will detect all the validators in their `validators_dir`.

Users will have issues if they are:

1. Using `--strict`.
1. Have keystores in their `~/.lighthouse/validators` directory that weren't being detected by the current keystore discovery method.

For users with (1), the VC will refuse to start because the `--strict` flag has been removed. They will be forced to review `--help` and choose an equivalent flag.

For users with (2), this seems fairly unlikely and since we're only in testnets there's no *real* value on the line here. I'm happy to take the risk, it would be a different case for mainnet.

## Additional Info

This PR adds functionality we will need for #1347.

## TODO

- [x] Reconsider flags
- [x] Move doc into a more reasonable chapter.
- [x] Check for compile warnings.

validator_client/Cargo.toml

@@ -24,6 +24,7 @@ types = { path = "../consensus/types" }
 serde = "1.0.110"
 serde_derive = "1.0.110"
 serde_json = "1.0.52"
+serde_yaml = "0.8.13"
 slog = { version = "2.5.2", features = ["max_level_trace", "release_max_level_trace"] }
 slog-async = "2.5.0"
 slog-term = "2.5.0"
@@ -44,3 +45,5 @@ tempdir = "0.3.7"
 rayon = "1.3.0"
 validator_dir = { path = "../common/validator_dir" }
 clap_utils = { path = "../common/clap_utils" }
+eth2_keystore = { path = "../crypto/eth2_keystore" }
+account_utils = { path = "../common/account_utils" }