feat: add service options

[reliveyy]

The PR removes .conf file usage from all images. And I also added serveral options --<service>.options to let the end users (include developers) to customize service launching command-line arguments.

Resolves #405

How to test

bash xud.sh -b service-options --geth.options="--cache 2048"

The custom options were applied in two ways

1. Inject into an environment variable like "XUD_OPTS"
2. Append by "$@"

Known issues

• A separated PR for removing xud.conf (see chore: remove xud.conf #722)
• A separated PR for removing lnd.conf (see chore: remove lnd.conf #723)
• Move options from service images' entrypoint.sh into utils
• A separated PR for editing service options without building new images. (TBD) If the option specified in --<service>.options exists in default settings and some program may not allow overriding previous options, then it could throw an error

[kilrau]

Looking at this I have the following concerns as discussed with @reliveyy :

• one needs to build a new image to apply a new option
• one can only append options
• logic looks complicated and somewhat error prone

Especially the first one is not acceptable for users in my opinion.

What we really want is: the user to be able to modify an option via mainnet.conf and via xud.conf/lnd.conf and xud-docker to apply these options. In order to allow manipulating xud-docker's mainnet.conf and e.g. lndbtc's lnd.conf both, which have an intersecting set of options, like mode = light equals bitcoin.node = neutrino all we should need to do is: apply mainnet.conf options as cli parameters (which by default supersede config files) and the rest is read from the e.g. lnd.conf file. Not sure if made myself clear :/