refactor!: environment variables validation

Refactor environment variable validation rules. Apply consistent
patterns to env validation.

The main motivation for this change was as follows:

1. Make env validation more strict and consistent. Stick to the same
validation patterns in different projects.

2. Provide consistent fallback options for optional variables.

Here are the main changes:

1. Previously if a user set an empty string as the variable name in the
.env file like this: ``` VARIABLE_NAME= ``` not all variable
initializers assigned a correct default value to the variable in this
case. Now it is fixed. For the purpose of this improvement, now all
optional variable initializers always have the `@Transform` decorator
that provides a default value for the variable. Add the two special
functions (`toNumber` and `toBoolean`) that support this change.

2. Make validation of boolean variables more strict. CAUTION: This might
be a breaking change for users who use values like "yes" to initialize
boolean variables.

3. Now the `ETH_NETWORK` variable is validated not only when the
validator source is "Lido". Make validation of this variable more
strict. CAUTION: Currently the EVM doesn't officially support testnets
other than those listed in the `Network` interface. But this might be a
breaking change if some users use EVM for other testnets.

4. Implement the new `toArrayOfUrls()` function to provide a reasonable
default value for variables that should have a list of URLs in their
values. This function also fixes a bug with default values for such
variables. Previously, if a user set an empty string as a value of such
variables, the validation worked incorrectly.

5. Now we validate that values in `EL_RPC_URLS`, `CL_API_URLS`, and
`VALIDATOR_REGISTRY_KEYSAPI_SOURCE_URLS` are indeed URLs.

6. The validation engine now removes trailing slashes from URL values.

7. More consistent validation for the `HTTP_PORT` and `DB_PORT` values.

8. Add `@Optional` decorator to all optional variables.

9. Change `@IsNumber` to `@IsInt`.

10. Move enums from the `env.validation.ts` to
`environment.interface.ts`.