community[patch]: support fastembed v1 and v2 (#19125)

**Description:**
#18040 forces `fastembed>2.0`, and this causes dependency conflicts with
the new `unstructured` package (different `onnxruntime`). There may be
other dependency conflicts.. The only way to use
`langchain-community>=0.0.28` is rollback to `unstructured 0.10.X`. But
new `unstructured` contains many fixes.

This PR allows to use both `fastembed` `v1` and `v2`.

How to reproduce:

`pyproject.toml`:
```toml
[tool.poetry]
name = "depstest"
version = "0.0.0"
description = "test"
authors = ["<dev@example.org>"]

[tool.poetry.dependencies]
python = ">=3.10,<3.12"
langchain-community = "^0.0.28"
fastembed = "^0.2.0"
unstructured = {extras = ["pdf"], version = "^0.12"}
```

```bash
$ poetry lock
```

Co-authored-by: Sergey Kozlov <sergey.kozlov@ludditelabs.io>

libs/community/langchain_community/embeddings/fastembed.py

@@ -57,13 +57,15 @@ class FastEmbedEmbeddings(BaseModel, Embeddings):
     @root_validator()
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that FastEmbed has been installed."""
-        try:
-            from fastembed import TextEmbedding
-
         model_name = values.get("model_name")
         max_length = values.get("max_length")
         cache_dir = values.get("cache_dir")
         threads = values.get("threads")
+
+        try:
+            # >= v0.2.0
+            from fastembed import TextEmbedding
+
             values["_model"] = TextEmbedding(
                 model_name=model_name,
                 max_length=max_length,
@@ -71,8 +73,19 @@ class FastEmbedEmbeddings(BaseModel, Embeddings):
                 threads=threads,
             )
         except ImportError as ie:
+            try:
+                # < v0.2.0
+                from fastembed.embedding import FlagEmbedding
+
+                values["_model"] = FlagEmbedding(
+                    model_name=model_name,
+                    max_length=max_length,
+                    cache_dir=cache_dir,
+                    threads=threads,
+                )
+            except ImportError:
                 raise ImportError(
-                "'FastEmbedEmbeddings' requires 'fastembed==v0.2.0' or above. "
+                    "Could not import 'fastembed' Python package. "
                     "Please install it with `pip install fastembed`."
                 ) from ie
         return values