Perform Base64 decode before JSON validation in create payloads

[ks-temporal]

What changed?

Previously, the Base64 decoding was done after JSON validation. This prevented
setting --input option with Base64 of JSON value. Base64 decoding is now done
before any JSON validation, if encoding indicates json.

Why changed?

The --input-base64 boolean flag indicates that the input is in base64. From help:

--input-base64     Assume inputs are base64-encoded and attempt to decode them.

If the flag is present, then input is not in JSON, and must be supplied as base64
encoded value. If no input-meta is supplied to change the encoding, or is set as
json/plain or similar, then previously, it attempted to validate input as JSON before
base64 decode, which failed.

Checklist

Stability

• Breaking changes are marked with 💥 in the PR title and release notes
• Changes to JSON output (-o json / -o jsonl) are treated as breaking changes

Design

• This feature does not depend on Cloud-only APIs or behavior (it works against an OSS server)
• New commands follow temporal <noun> <verb> structure (e.g. temporal workflow start)
• New flags are named after the API concept, not the implementation mechanism (good: --search-attribute, bad: --index-field)
• New flags don't duplicate an existing flag that serves the same purpose
• New flags do not have short aliases without strong justification
• Experimental features are marked with (Experimental) in commands.yaml

Help text (see style guide at the top of commands.yaml)

• All flags shown in help text and examples are implemented and functional
• Summaries use sentence case and have no trailing period
• Long descriptions end with a period and include at least one example invocation
• Examples use long flags (--namespace, not -n), one flag per line
• Placeholder values use YourXxx form (YourWorkflowId, YourNamespace)

Behavior

• Results go to stdout; errors and warnings go to stderr
• Error messages are lowercase with no trailing punctuation

Tests

• Added functional test(s) (SharedServerSuite)
• Added unit test(s) (func TestXxx) where applicable

Manual tests

Setup

Start server, and worker from samples-go for standalone_activity/helloworld

temporal server start-dev --headless
cd samples-go
go run standalone-activity/helloworld/worker/main.go

Happy path

The following test should not work previously, but should work after the fix in this PR.

$ echo -e -n "\"Temporal\"" | base64
IlRlbXBvcmFsIg==

$ temporal activity execute \
    --type Activity \
    --task-queue standalone-activity-helloworld \
    --activity-id YourActivityId \
    --start-to-close-timeout 10s \
    --input IlRlbXBvcmFsIg== \
    --input-base64

Without the fix:

Error: input #1 is not valid JSON

With the fix:

Running execution:
  ActivityId  YourActivityId
  RunId       ...
  Namespace   default
Results:
  Status  COMPLETED
  Result  "Hello Temporal!"