Allow more/custom labels in `encodePemBlock`

[spkersten]

For example, in some cases, other parties require "EC PRIVATE KEY" as a label. Which is not one of the options in PemLabel.

[jonasfj]

It's possible to parse with unsafeIgnoreLabel: true.

Are these labels documented anywhere, I don't see them in:
https://www.rfc-editor.org/rfc/rfc7468#section-4

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I could see us allowing a custom string as label too... Maybe as an extra parameter, or as a separate constructor, or just as separate utility methods.

Finding a good place to stick in the label is not easy, maybe we shouldn't have used an enum in the first place.

On the flip side, if these cases are rare on non-spec'ed, maybe unsafeIgnoreLabel is the right choice.

[spkersten]

For parsing unsafeIgnoreLabel is fine. But my problem was with serializing.

[jonasfj]

Are there any specifications that suggest using a label like "EC PRIVATE KEY"?

It's possible that dumb and simple thing we should do is take a string in the constructor and deprecate the PemLabel enum.

Is it common to use labels not specified in RFC 7468. If there is a longer list of common labels, then we can extend the enum. If it's common then we probably need to support string.

For the record: a simple workaround is just .replaceAll('PRIVATE KEY', 'EC PRIVATE KEY') on the serialized output.

[spkersten]

OpenSSL seems to want "EC PRIVATE KEY".

The search and replace is what I've been using. It's just a bit ugly.