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. Also for this purpose,
the `toBoolean` function has been refactored to be more similar to the
`toNumber` function.

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. 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.

4. Now we validate that values in `PROVIDERS_URLS` and `CL_API_URLS` are
indeed URLs.

5. The `CHAIN_ID` variable now supports only a pre-defined list of
testnet IDs. Add the appropriate `Network` enum for this purpose.
CAUTION: Currently the Keys API doesn't officially support testnets
other than Goerli, Holesky, and Mainnet. But this might be a breaking
change if some users use Keys API for other testnets.

6. More strict validation for the `PORT` and `DB_PORT` values. CAUTION:
This might be a breaking change.

7. Add the `@IsNotEmpty` decorator to the required variables.

8. Fix confusing validation rules for the `DB_PASSWORD` variable. Add
default value.

Resolve review comments. The main changes are:

1. Loosen validation rules for boolean variables. Now numbers "1" and
"0" and constants "yes" and "no" are accepted as valid boolean values.

2. Make DB password value required. Remove the default empty DB password
value.

3. Rename the `Network` interface to `Chain`.