Add support for specifying the output directory from the command line

[lewisjared]

Description

Adds support for a --session_name CLI argument to allow users to specify the session name.
This allows the ability to write results to a fixed and known directory.

Closes #3100

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[bouweandela]

Thanks @lewisjared! This is a feature that several people have asked about over the years.

[bouweandela]

This doesn't render correctly when running esmvaltool run --help:

NAME
    esmvaltool run - Execute an ESMValTool recipe.

SYNOPSIS
    esmvaltool run RECIPE <flags>

DESCRIPTION
    `esmvaltool run` executes the given recipe. To see a list of available
    recipes or create a local copy of any of them, use the
    `esmvaltool recipes` command group.

    A list of possible flags is given here:
    https://docs.esmvaltool.org/projects/ESMValCore/en/latest/quickstart/configure.html#configuration-options

POSITIONAL ARGUMENTS
    RECIPE
        Path to the recipe to run.

FLAGS
    -s, --session_name=SESSION_NAME
        Type: Optional[]
        Default: None
        Name of the session output subdirectory (relative to the configured ``output_dir``). When given, this value is used instead of the default ``<recipe.stem>_<timestamp>``.

        If the given name already exists, a suffix is added to the session name to avoid overwriting existing data. **kwargs: Command line arguments to override configuration parameters.
    Additional flags are accepted.

NOTES
    You can also use flags syntax for POSITIONAL ARGUMENTS