feat: Add a way to skip context variable interpolation/injection.

### Is your feature request related to a specific problem?

Yes, I'm unable to include the string `${}` in my prompt. The interpolation code wants to expand it to the contents of a context key `` and remove the curly braces.

### Describe the Solution You'd Like

I'd like a way to turn off or escape curly brace expressions in prompt strings.

### Impact on your work

I'm working on something that requires describing the output format I'd like the LLM to produce, and part of that format includes curly braces. I have had to monkey-patch the ADK in order to get the functionality I need.

### Willingness to contribute

I would be willing to contribute a PR, although I don't have one prepared.

### Describe Alternatives You've Considered

I've tried escaping the curly braces with backslashes, doubling them, and including the the key `"" = "{}"` in the context, but escaping didn't work, and including the contents of every curly brace in the context isn't scalable.

### Proposed API / Implementation

#### Option 1: Support Escaping via Double Braces
Adopt the Python standard (used in str.format) where double braces `{{` and `}}` are resolved to literal `{` and `}` and ignored by the parser.

#### Option 2: Allow Custom Delimiters
Allow users to specify custom delimiters for templates to avoid collisions with the target template's syntax.