sampleconfig: Use embed with external files.

[davecgh]

This updates the sampleconfig package to make use of Go's embed functionality introduced in Go 1.16 which is acceptable since the containing module requires Go 1.19.

Embedded files are preferable because they bring the benefits of both external files in the repository that are easier to work with for external tools and the ability to access their content programmatically without having to deal with the various issues typically brought about by programmatically accessing external files.

Also, when this was last updated to change the exported variable for dcrd's sample config file over to a function (so it can't be mutated and the implementation details can be changed if needed), the case for dcrctl was missed. Moreover, the existence of both configs makes the func name FileContents ambiguous.

Unfortunately, those things can't be changed now without causing a major module version bump to the main dcrd module which we really want to avoid.

So, in order to address those things without needing a major module version bump, this deprecates the FileContents func and DcrctlSampleConfig variable and introduces two new functions named Dcrd and Dcrctl that return the respective embedded sample configs.

[jholdstock]

Nice improvement. README can be updated.

Package sampleconfig provides a single function that returns the contents of the sample configuration file for dcrd.

[davecgh]

Nice improvement. README can be updated.

Nice catch. It was previously updated because there was only intended to be a single function, but alas, since the dcrctl bits were missed there are now multiple. Updated.