[GEOS-11246] Schemaless plugin performance improvement

[Mike7311]

https://osgeo-org.atlassian.net/browse/GEOS-11246

Changed DynamicComplexTypeImpl to search attributes in LinkedHashMap instead of ArrayList for Mongo schemaless plugin.
For my particular use case (969 documents with deep nesting) ordinary GetFeature request time (without any filter) decreased from 12-14 seconds to 2-5 seconds.

Checklist

• I have read the contribution guidelines.
• I have sent a Contribution Licence Agreement (not required for small changes, e.g., fixing typos in documentation).
• First PR targets the main branch (backports managed later; ignore for branch specific issues).
• All the build checks are green (see automated QA checks).

For core and extension modules:

• New unit tests have been added covering the changes.
• Documentation has been updated (if change is visible to end users).
• The REST API docs have been updated (when changing configuration objects or the REST controllers).
• There is an issue in the GeoServer Jira (except for changes that do not affect administrators or end users in any way).
• Commit message(s) must be in the form [GEOS-XYZWV] Title of the Jira ticket.
• Bug fixes and small new features are presented as a single commit.
• Each commit has a single objective (if there are multiple commits, each has a separate JIRA ticket describing its goal).