Metadata explorer: decoding options and richer viewing for CustomDebugInformation blobs

[siegfriedpammer]

Follow-up to #3747 (@fowl2), which decodes EmbeddedSource blobs into the row-details area of the CustomDebugInformation table. From the PR description:

Future: It would probably be nice if there was some way to select/override the decoding method (eg. force a specific character encoding / base64 / hex), open in a tab, add syntax highlighting, save the raw content, etc.

Concretely, for the blob row-details viewer:

• Select/override the decoding method (force a specific character encoding, base64, hex)
• Open the decoded content in a document tab
• Syntax highlighting for decoded embedded source
• Save the raw blob content to a file