[fix] cache loader cannot be serialized

[gnehil]

Proposed changes

Since CacheLoader does not implement the Serializable interface, the anonymous inner class used to construct LoadingCache cannot be serialized.
The internal class BackendCacheLoader inherits CacheLoader and implements Serializable, which solves the problem that the anonymous internal class of CacheLoading cannot be serialized.

Issue Number: close #xxx

Problem Summary:

Describe the overview of changes.

Checklist(Required)

1. Does it affect the original behavior: (Yes/No/I Don't know)
2. Has unit tests been added: (Yes/No/No Need)
3. Has document been added or modified: (Yes/No/No Need)
4. Does it need to update dependencies: (Yes/No)
5. Are there any changes that cannot be rolled back: (Yes/No)

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[hf200012]

LGTM