add base64 and dict_merge extension

[gebinic]

Added an extension which encodes and decodes a string. Supported unix and windows systems.

EDIT: add dict_merge extension

[nicks]

thanks for the contribution!

i'm fine with the base64 extension

i'm nervous about having the dict_merge extension in the main extensions repo, because:

• starlark already has a dictionary union operator (see: https://github.com/bazelbuild/starlark/blob/master/spec.md#dictionaries)
• historically, deep merge has been pretty controversial, because it's hard to make the behavior intuitive

[gebinic]

We distinguish between deploy_config.yml ("global" configuration file) and local_config.yml (git ignored file, only local user configuration file). Due to our configuration model, it is extremely difficult to merge these configuration files manually, among other things with the dictionary union operator. Therefore, we decided to introduce the dict_merge function, which simply merges all configurations deeply. I think this could be a use case that also affects other tilt users.

[nicks]

i think we'd accept a pr for just the base64 but wouldn't accept dict_merge, for the reasons stated above. closing this pr, but feel free to repopen for the base64 part