[buildozer]: Support adding symbols instead of literal strings

[justhecuke]

If I want to, for example, add a symbol with a value to a list, that is not possible right now.

buildozer 'add tags SOME_CONST' //pkg:target
-->
some_rule(
  name = "target",
  tags = ["SOME_CONST"],
  ...
)

What I want is the ability to do something such as:

buildozer --symbol 'add tags SOME_CONST' //pkg:target
-->
some_rule(
  name = "target",
  tags = [SOME_CONST],
  ...
)

and

buildozer --symbol 'set tags LIST_CONST' //pkg:target
-->
some_rule(
  name = "target",
  tags = LIST_CONST,
  ...
)

In this way, we can use buildozer better for larger refactors if we want to centralize control of a few key attributes without needing to create a mess of macros and rules.

[stagnation]

Nice idea! I have historically wanted all tags to be included as literals, to have a centralized location with documentation for what they all mean. And to reduce the risk of typos that are very hard to catch after the fact.
At one point I tried to do a post-process sed to make them literals after the fact, I think that should work. I did not look into the tables configuration to temporarily define tags as not-strings either, though maybe that works.

[tetromino]

I'd suggest expanding this FR to support adding arbitrary non-string values - for example, structs, dicts, etc., as well as raw symbols.

(There are various tricks you can do with quoting/unquoting, but it would be good to have a solution that works properly out of the box.)

@Jimilian FYI