Read map value from session

### Related Problem or Business Case

See https://github.com/frankframework/frankframework/pull/11382

This needs to be used for more things. Headers, multiparts, jms properties, etc

We might need to reserve the keys for these maps and validate they are not used by other parameters.

See:
https://github.com/frankframework/frankframework/blob/d6d1f5e4483545100dd41176a02ad0769b036c20/core/src/main/java/org/frankframework/stream/MessageContext.java#L49-L51
Used as such:
https://github.com/frankframework/frankframework/blob/d6d1f5e4483545100dd41176a02ad0769b036c20/core/src/main/java/org/frankframework/soap/SoapContext.java#L97-L102
https://github.com/frankframework/frankframework/blob/7a89be4d06334bb60ba1e0d3bb711845002a560b/core/src/main/java/org/frankframework/util/TransformerPool.java#L522

### Describe the Feature

It should be easy to add a new value to an inner-map. Ideally without breaking existing code.

### Collaboration with External Companies

No, community resolution only

### Anything else?

_No response_